8 replies to this topic

[AdamR]

Can anyone explain why different foods have different cook temperatures?

 

In particular, many recommend that cooking fish to 63°C will render it safe.  I understand that different pathogens are associated with different foods, and that different pathogens have different kill temps.

 

What I don't understand is that Listeria M has a kill temp of 70°C for 2 minutes, and this bacteria is often associated with fish.   Therefore, why is 63°C an accepted safe temp for fish?

 

Any ideas?

 

 

 

[qui]

It's all about the critical temperature limit where the bacteria can growth or keep alive in potentially hazardous foods, this range temperature is between 5- 60 Celsius Degrees.

It depends on the type of food, you can know which pathogens are likely to growth in, and therefore you can choose the adequate temperature for cooking, there are guidelines on the Principles and Applications of HACCP , the Limiting Factors.

For example for Listeria, at 85 celsius degrees there is a 100% of Lethal rate, time 0.02 min, but that might be too high; at 70 degrees, the lethal rate is 1.00 for 2 min, but it acceptable 63 degrees for 17 min with a lethal rate of 0.117

[Charles.C]

Can anyone explain why different foods have different cook temperatures?

 

In particular, many recommend that cooking fish to 63°C will render it safe.  I understand that different pathogens are associated with different foods, and that different pathogens have different kill temps.

 

What I don't understand is that Listeria M has a kill temp of 70°C for 2 minutes, and this bacteria is often associated with fish.   Therefore, why is 63°C an accepted safe temp for fish?

 

 

 

Dear AdamR,

 

Yr first and last queries relate to organoleptic quality (OLQ) / consumer health safety and safety respectively. In fact yr last query is a partial mis-statement.

For OLQ, I daresay a chef is the best responder?. Plus a food nutritionalist maybe?.  And the consumer?.

 

The safety part is related to the minimum temperature required for inactivation of a bacterial species plus the fact that the inactivation is a kinetic process, eg a function of temperature, time, additional parameters.

For example, a typical cell of L.mono. in a given environment will take longer to be inactivated at 63degC (eg the previous post) compared to an applied temperature of 70degC.

 

Yr first "safety" query also relates to the “additional parameters” above. For an example of  one directly related, intrinsic factor –

 

Many non-sporulating bacteria are readily inactivated at temperatures of about 50degC and above, the rate of inactivation increasing as the temperature is raised. By contrast, bacterial spores are much more resistant and moist heat temperatures of at least 100degC and often appreciably higher are usually needed to produce significant decreases in viability.

(the reason is that the underlying mechanisms of inactivation are different).
 

Other factors may include -  the composition and pH of the “system”, the type of organism (there could be a strain-dependent response), the growth conditions (medium, growth phase, temperature), heating method (open systems are less accurate than closed ones) and the recovery conditions.

 

Rgds / Charles.C

[AdamR]

Thanks for the info.  My query is related only to safety.  What I can't understand is shown in the following example:

 

A raw chicken could possibly be contaminated with Listeria M, among other pathogens.  A recommended safe cooking temp is often stated as 75°C.

A raw fish can also possibly be contaminated with Listeria M, among other pathogens.  Yet many sources say that cooking it to only 63°C will render it safe to eat.

 

I don't suppose many will cook it to 63°C for 17 mins if say grilling or frying fish. Therefore how is the Listeria M inactivated and how is it safe to eat?

 

Apologies if this has been answered above and I haven't understood!

[Charles.C]

Dear AdamR,

 

We seem to have rather veered away from the original title thread. No problem.

 

A raw fish can also possibly be contaminated with Listeria M, among other pathogens.  Yet many sources say that cooking it to only 63°C will render it safe to eat.

 

Is it possible that yr "sources" have a different interpretation as to the word "safe" as, perhaps, compared to the FSA?

 

Perhaps yr sources were not referring to risks associated specifically with L.mono. ? For example, i daresay the USA focus more on salmonella.

 

 

I don't suppose many will cook it to 63°C for 17 mins if say grilling or frying fish. Therefore how is the Listeria M inactivated and how is it safe to eat?

 

Well, if the original level of L.mono in the fish was 10^9 / gm, it probably wasn't safe to eat, even if cooked for 17mins at a core temperature of 63degC. Unless perhaps you only had one spoonful or have a particularly good immune system? (I haven't looked up the relevant data).

.

The point is that the guidelines are usually intended to be conservative and make certain (probabilistic?) assumptions as to what constitutes a "safe" product. And the requirements for its achievance. Failure to observe the guidelines presumably yields a higher probability of not delivering a "safe" product,  in some cases much higher.

 

It is well-documented that official assumptions have on occasion been insufficiently conservative, eg hamburgers in USA.

 

I just looked in my 1978 book on Theory of Catering and noticed this sentence -

 

It is important to remember that that it is necessary not only to heat foods to a sufficiently high temperature but also for a sufficient length of time to be sure of safe food.

However no specific numbers are given.

 

Like yourself, i have also seen temperatures in the mid-60's (degC) recommended from an "eating" POV with, from memory, no specific times attached. Maybe the fish were straight from the sea.

 

Rgds / Charles.C

 

PS (added) - I am assuming you are familiar with basic, heat induced, bacterial reduction formulae and the logic underlying the existence of the 2min/70degC rule /  its lethality equivalence to other pairs of time/temperature conditions as in tables frequently linked on this forum.

[George @ Safefood 360°]

The temperatures usually quoted are based on equivalents. Specific temperature at a specific time and or the equivalents. Higher temperature - shorter times etc.

 

George 

[SUSHIL]

Hello Mr AdamR,

 

Thermal resistance of Listeria monocytogenes

The heat resistance of L. monocytogenes is influenced by many factors such as strain variation, previous growth conditions, exposure to heat shock, acid, as well as other stresses, and composition of the heating menstruum .

D-value is used to describe the heat resistance of a certain strain at a certain temperature as it is the time needed to destroy 90% of cells at that temperature. z-value is the temperature difference required to destroy 90% of the bacteria with a 10-fold change in heating time.

The strain variation of 21 L. monocytogenes strains at 55°C in BHI broth at pH 6 was 4.7 fold (23.8 to 111 min) shown by the D55°C-values. L. monocytogenes isolate from brine cheese had 1.5 to 2.8 fold higher D56-value than the three listeriosis outbreak strains tested.
Cells in stationary phase of growth appear to be the most resistant to thermal stress . Stationary phase cells of L. monocytogenes had 2.8 to 5.6 times higher D60°C-values (0.45 to 12.5 min) than the logarithmic phase cells, each tested in three different media (minced beef, tryptic phosphate broth TPB and TPB supplemented with 8 g/l lactic acid) at four different pH from 5.4 to 7. L. monocytogenes Scott A strain had 7.6 times higher D56°C-value at the early stationary phase than at the exponential phase.

The composition of the growth medium, whether a food or laboratory culture broth, affect rates of growth and the synthesis of cellular constituents that determine the thermal tolerance of bacterial cells. The D60°C-values for L. monocytogenes is 2 to 6 fold higher in minced beef than in TPB . In half cream, double cream and butter the D-values of two L. monocytogenes strains were 1.1 to 8 fold higher than in TSB also indicative of notable differences in D-values between food products .

 

The environment in which cells are grown can be a major determinant of their heat resistance.
L. monocytogenes –strain Scott A growing in tryptic phosphate broth containing 0.09, 0.5, 1.0 or 1.5 M NaCl was heated in media with the same salt concentrations resulting in 4 log reduction at 60 °C in 1.6, 2.5, 7.4 and 38.1 min, respectively .
Increased heat resistance is also induced by starvation, low pH, and addition of antimicrobial compounds like ethanol, or hydrogen peroxide to the growth media.
The growth temperature affects the heat resistance and in general the cells grown at higher temperatures are more heat resistant than those grown at lower temperatures.
The rate at which cells are heated during testing also influences their survival. When cells are heated slowly, they exhibit a greater heat resistance than when heated rapidly
Heat-shock, a short-term exposure of cells to temperatures above the optimum growth, results in increased heat resistance. The degree of enhanced thermal resistance is strain dependent and also varies with the length of the heat shock, the pH of the medium, and the growth phase of the cells. The maximum increase in thermotolerance was 4 and 7 fold for a L. monocytogenes strain previously grown at 37 °C and 4 °C and heat-shocked at 45 °C and at 47.5 °C, respectively. On the other hand cold-shock decreases the heat
resistance of L. monocytogenes Increased heat tolerance can also be induced by short-term exposure to high salt or solute levels . Decreasing aw values and increasing solute concentrations result in greater heat resistance in L. monocytogenes.
The presence of 10% salt or aw<0.92 resulted in a high heat resistance and it became the most heat resistant vegetative pathogen .

 

Hence validation tests are to be carried out for all food products

Attached Heat Resistance of listeria doc

 

 

 

Attached Files

•  Heat resistance of L. monocytogenes. .docx   22.16KB   39 downloads

[Charles.C]

Dear Sushil,

 

I agree with you that there are many factors involved in establishing appropriate D and z values in a given situation.

 

Different locations may also "justify" different viewpoints / criteria as the basis for their safety assessment.

 

Rightly or wrongly, any local regulatory controls will act as the validation.

 

Just like microbiological specifications.

 

Rgds / Charles.C

[campbell]

The main doifferences are that cooking will change texture and set proteins etc to give the visual appearance of accptability. However, for safety you will need to look at microbiolgical data. 

For example your cooking at 60-65 will set protein etc but 70 for 2 minutes will kill the bacteria present,